I have worked extensively with Rapattoni systems including integrating their RETS system. For an example of this, check out www.johnlscott.com.

Not all MLS's subscribe to the RETS interface. Check here to se if your MLS is listed: http://www.rapattoni.com/supp/mls/rets/rets_servers.asp

You should contact your MLS to find out what other options are available. FTP and RETS access are both frequently used by Rapattoni MLS's.

If you have any other questions, feel free to post them here.